Radiotherapy (RT) is a therapeutic procedure based on ionizing radiation for the treatment, for example, of cancer. Radiotherapy may, however, also be used for the treatment of other disorders. With radiotherapy, the attempt is made to conduct an adequate therapeutic radiation dose to a diseased area of tissue, while surrounding healthy tissue is spared. The therapeutic effect is based on an ionizing effect of the radiation on diseased tissue.
The dose application by radiation takes place, for example, by intensity-modulated photon therapy (IMRT), protons or carbon ions. A precondition for this is radiation treatment planning based on three-dimensional diagnostics, such as computed tomography or magnetic resonance tomography. In the radiation treatment planning, the radiation parameters are determined such that the necessary total radiation dose is applied in the volume of the tumor or the target volume, and surrounding healthy tissue is spared as may best possibly be achieved. For example, the application of radiation may be carried out by photon radiation in a “Step-and-Shoot” arrangement, in which the beam is switched off while laminations of a laminated collimator move in order by the movement, to define a subsequent beam section. A further arrangement is referred to as the dynamic technique, in which the beam remains switched on while the laminations move. For protons or heavy ions, either “scanned-beam” techniques, in which the beam is scanned in a grid pattern over a target volume, are used, or passive field-shaping techniques are used (e.g., use is made of compensators and energy modulators and range modulators, respectively).
The planning of the radiation treatment of a target volume (e.g., of a tumor) takes place within the framework of a treatment plan. A treatment plan includes, for example, the number and orientation of several different beams that are required for the application of a specific prescribed dose (e.g., a total radiation dose) in the target volume. In each volume element, a beam may apply a part radiation dose. A number of techniques exist in order to vary the applied dose for each volume element.
For dosimetric reasons, more than one beam may be used in order to apply the total radiation dose that has been calculated in relation, for example, to the type of the tumor. For example, the preparation of a treatment plan may include the definition of organs at risk (OAR) that are to be protected from the application of a radiation dose, since OARs react with particular sensitivity to radiation, and major undesirable side-effects are possible in the event of the OAR being irradiated. The radiation treatment may also include the irradiation of several target volumes. For each target volume, the calculation of optimum radiation treatment parameters (e.g., number, type, intensity distribution, and energy of different beams, etc.) is then carried out in an iterative optimization process. Individual parameters from among these may also be predetermined in order to reduce the complexity of the problem.
In certain clinical cases, more extensive methods are provided in order to make possible the treatment of the patient by radiotherapy. Such a case is the situation in which the projection of a target volume in the direction of a beam is greater than the maximum irradiation field of vision of a beam. For example, the maximum field of vision of a beam may be defined by hardware-side limitations. A beam is then to be split into two or more beams. This procedure may be referred to as a “beam split,” where the splitting may be achieved, for example, by a “split plane” being defined in order to distinguish on which side of the split plane a specific beam applies a part radiation dose. In the literature, methods that allow for the splitting of a beam are known. For example, Q. Wu et al. disclose in Phys. Med. Biol. 45 (2000) 1731-1740 a method for the splitting of beams.
A further clinical area, in which more extensive methods are provided for the treatment of the patient using radiotherapy, is dosimetric optimization. If a dosimetric advantage is anticipated, it may be advantageous for the target volume to be split into different sub-volumes. The splitting is not necessarily carried out on the basis of hardware limitations, as has been described with regard to the split plane. The different sub-volumes are then irradiated by different beams or several beams in each case. For example, in “scan particle beam therapy,” such a splitting of the target volume into sub-volumes is used. This area of application is referred to as the “beam patch,” and the corresponding control planes are referred to as patch planes. For example, E. B. Hug et al. disclose in Int. J. Radiat. Onkol. Biol. Phys. 47 (2000) 979, a method of scan proton beam therapy, in which a single beam is moved by scanned magnets into the desired position. Advantages that derive from the use of two proton beams, set in relation to each other by a patch plane are shown for a class of appropriate clinical cases. For example, it is shown that an OAR may be exempted from the application of a dose.